Z-source, quasi-Z-source and trans-Z-source inverters are new DC/AC (voltage) converters in power electronics. These make it possible to both increase and decrease input voltage without additional switching elements. These converters consist of an LC circuit (of Z-source, quasi-Z-source or trans-Z-source-type) and a single-phase or multi-phase inverter (FIG. 1). The voltage amplitude is increased by a specific shoot-through state that is generated by turning on the switching elements of one bridge arm of the inverter simultaneously. In this case, the energy of the magnetic field is accumulated in the chokes of LC circuits without short-circuiting the capacitors during the shoot-through. The accumulated energy is used to increase the voltage of the DC-link during the traditional conductivity phase that follows the shoot-through. Such inverters are meant for various power electronics applications where flexible control and reliability of a device are especially important.
So far, Z-source, quasi-Z-source and trans-Z-source inverters have been controlled using sine wave modulation with shoot-through states, i.e. pulse-width modulation where the aim is to shape the output voltage in a way that near-sine current is generated. Typically, shoot-through is generated during zero states. As a result, shoot-through zero states occur whereby active states remain constant and zero states are reduced by the amount of shoot-through. Sine wave modulation with shoot-through states has mainly been used to control electric motors and link alternative and renewable energy sources to AC-power grids. The disadvantages of sine wave modulation are its rather complicated control algorithm and the fact that low-frequency sine signals cannot pass through a high-frequency pulse transformer in applications requiring galvanic insulation. To solve the problem, Z-source, quasi-Z-source, and trans-Z-source inverters are controlled using modified sine wave modulation methods with shoot-through states: pulse-width modulation (PWM) or phase-shift modulation (PSM). In both these methods the goal is not to generate sinusoidal current but a simple square wave signal with fixed duty ratio that can most efficiently pass an insulation transformer. And the control algorithm is simpler and burdens the control system less. A shoot-through state is added to the modified sine wave signal and the relative duration of the shoot-through determines the voltage amplitude of the DC-link. Shoot-through states are distributed across the period in a way that the number of higher harmonics would be minimal in the output voltage of the inverter. To reduce the switching and conductivity losses, the number of shoot-through states per switching period is limited to two and the shoot-through current is distributed equally across the transistors of both arms of the inverter.
In the case of PSM, the pulse width is kept constant. The output of the inverter is adjusted by changing the mutual phase angle of the control signals and the duration of a shoot-through (FIG. 5). In the case of PSM, a shoot-through is generated during zero states. The prerequisite is that the duration of the zero state (tZ) is longer than the maximum duration of the shoot-through state. The switching period consists of three parts: an active state, a shoot-through state and a zero state. In contrast to the PWM method, where one group of switching elements (either the upper or lower one) constantly generates zero states, here, the upper and lower switching element groups take turns to generate a zero state. This ensures equal operating frequency of the switching elements. If the operating frequency of switching elements is equal, the switching losses are equal as well and the switching elements have an equal load.
Patent US005784267A concerning a modified sine wave method is already known. An AC converter is controlled using a modified sine wave method without shoot-through states. The disadvantage of this method is that the voltage cannot be increased.
Also, a Z-source inverter described in US2009066271 is known. The Z-source inverter is used for generating a three-phase sine voltage. The voltage is increased by shoot-through states integrated into the control algorithm. The disadvantage of the described solution is that it applies only to sine wave modulation and does not determine the methods for shoot-through generation.
Furthermore, a current source inverter described in WO9421021 is also known. Some switching elements, a capacitor and two chokes are added to a three-phase current source inverter to achieve soft switching. The inverter operates using sine wave modulation. The disadvantage of the inverter is that it does not enable to generate freewheeling, zero or shoot-through states.